I. Field of the Invention
The present invention relates generally to the fields of pharmacology. More particularly, it provides cDNA encoding novel high affinity choline transporters, methods for their use in screening and therapy.
II. Description of Related Art
The molecular mechanisms involved in neurotransmission in the central nervous system is a multi-step process involving the release of neurotransmitter from the presynaptic terminal, diffusion across the synaptic cleft, and binding to receptors resulting in an alteration in the electrical properties of the postsynaptic neuron. For most neurotransmitters, transmission is terminated by the rapid uptake of neurotransmitter by specific, high-affinity transporters located in the presynaptic terminal and/or surrounding glial cells (Kanner et al. 1987). Since inhibition of uptake by pharmacologic agents increases the levels of neurotransmitter in the synapse, and enhances synaptic transmission, neurotransmitter transporters provide important targets for therapeutic intervention. (U.S. Pat. No. 5,658,786).
In the central and peripheral nervous system, cholinergic neurons regulate a variety of autonomic, cognitive and motor functions (Fibiger, 1991). Dysfunction of cholinergic signaling has been implicated in Alzheimer's disease (Coyle et al., 1983), Parkinson's disease Calabresi et al., 2000), schizophrenia (Tandon, 1999), Huntington's disease (Lange et al., 1992) and dysautonomia (Baron et al., 1996).
Okuda et al. (2000) have identified and characterized a high-affinity choline transporter in the rat. However, using human gene products in drug development offers significant advantages over those of other species, which may not exhibit the same pharmacological profiles as human genes. Reagents developed to interfere or modulate the rat transporter may not be as clinically relevant as reagents developed using a human system. Also, since no cell lines exist which express the human choline transporter, lack of crossreactivity must be validated using less ideal tissue, for example, postmortem brain tissue. Okuda et al. (2000) provides no sequence relationship between rat and human choline transporters. Also, the Okuda's proposed topology for rat choline transporters is distinct from the topology as described herein. Okuda et al. proposes 12 transmembrane domains for rat and C. elegans choline transporters. However, human (Apparsundaram et al. 2000) and mouse (Apparsundaram et al., Biochem. Soc. Trans. 29:711-6, 2001) choline transporters were not described, and these transporters have a distinct protein topology (13 transmembrane domains) based on the sequence relationship of cDNAs to related Na+/glucose family of transporters.
To facilitate a human-target based approach to drug design, a human ortholog of CHT has been cloned. A mouse ortholog of CHT also has been cloned. These genes will be beneficial in drug screening and animal modeling of cholinergic function.